Overvoltage arrestor having a light dispersion of fine metallic dust on its inside walls



B. G. A. LINDGREN Dec. 22, 1964 OVERVOLTAGE ARRESTOR HAVING A LIGHTDISPERSION OF FINE METALLIC DUST 0N ITS INSIDE WALLS Filed March 1, 1961ALUMINUM DU$T United States Patent f 3,162,741 OVERVOLTAGE ARRESTORHAVING A LIGHT DESPERSIQN OF FINE METALLIC DUST ON ITS INSIDE WALLS B0Gustav August Lindgren, Johanneshov, gweden, assignor toTelefonaktiebolaget L M Ericsson, Stockhelm, Sweden, a corporation ofSweden Filed Mar. 1, 1961, Ser. No. 92,570 Claims priority, applicationSweden Mar. 11, 1960 Claims. (Cl. 200-1135) The present invention refersto an electric overvoltage protector in the form of a gas dischargetube.

With gas discharge tubes in general it is earlier known to provide abridge or similar device consisting of some convenient electricallyconducting material, in parallel to the discharge gap in order to reducethe ignition delay, said bridge being preferably located on the tubeenvelope. Furthermore this bridge is generally electrically connected toone of the tube electrodes, whereby a discharge appears between thebridge and the one electrode not electrically connected thereto, at themoment the tube is connected, whereafter the main discharge gap isignited.

With gas discharge tubes of the type used as overvoltage protectors itis of vital importance that the tube has an ignition delay as short aspossible in order to react very quickly to sudden shock voltages. Tothis end it has earlier been proposed to provide an auxiliary electrodein the form of a bridge in parallel to the discharge gap, and thisbridge is preferably provided on the inside of the tube envelope in theform of a coating as in other types of discharge tubes, except that thecoating must not be conducting, in any case not at lower voltages, andfurthermore is preferably electrically insulated from the mainelectrodes. Such bridges have been prepared by metallic evaporation, sothat a thin conducting layer is formed which subsequently is exposed toa heavy current shock and thereby divided into a multitude of insulatedislands, so that a number of microscopic sparking gaps are obtained,whereby the electrons and ions, liberated at discharges in these gaps,cause a pre-ionizing of the main discharge gap. Another bridge typeearlier proposed involves a purely mechanical mixture of at least twocomponents, one of which is a conductor, the other being an insulator,which mixture gives a non-linear, voltage dependent resistancecharacteristic. This last mentioned type of bridge type has considerablybetter properties compared to the first-mentioned type, but hasnevertheless certain practical disadvantages specifically in connectionwith the pure mechanical structure of the tube and the application ofthe bridge to the tube envelope.

These disadvantages are eliminated by the present invention by providinga new mechanical structure of the tube and an entirely new means forobtaining the desired reduction of the ignition delay. The inventionresides basically in an overvoltage protector or arrestor in the form ofa gas discharge tube including two generally parallel bimetalliccontacts. These contacts define a discharge gap therebetween which isshortcircuited when the contacts are heated. The contacts are coatedwith a light radioactive coating on surface areas facing each other anda light dispersion of a fine dust of a light metal such as aluminum isprovided on the inside walls of the discharge tube. The radioactivecoating on the contacts causes an ionization of the gap space betweenthe coated areas of the contacts so that a discharge will occur betweensaid areas only, in response to a suddenly appearing excess voltage. Thecoating affords the further advantage that it protects the metalsurfaces of the contacts during the discharge. The discharge current maymomentarily reach a magnitude of several thousand amperes. A dischargeof such magnitude, if occurring between uncoated metal sur- 3,162,741?atented Dec. 22, 1964 faces may tear off metal particles from suchsurfaces and from a metal coating on the inside walls of the dischargetube, known as the Edison effect. The result would be such a lowering ofthe insulating resistance between the electrodes that the tube is nolonger usable.

The light metal dust may be introduced into the tube in any suitablemanner. The individual particles, due to the insignificant weight andmass thereof will adhere themselves to the inside walls of the tube bythe inherently present force of attraction. The particles form amultitude of minute spark gaps which will cause an additional ionizationof the main discharge gap between the contact electrodes.

The invention will be further described with reference to the singlefigure of the attached drawing, said figure showing an embodiment of anovervoltage protector in accordance with the invention.

The exemplified arrestor comprises a tube 1 having a base 2. In the basethere are provided two contact tags 3 and 4 which are electricallyconnected to two bimetallic electrodes 5 and d. The contact tags andalso the bimetallic electrodes are secured in limit means in the form ofa ceramic bushing 7 which is provided inside the tube in connection withthe tube base. The bimetallic electrodes 5 and 6 are at their free endsprovided with molybdenum bars 8 and 9, respectively, which are arrangedperpendicularly to each other on the respective electrodes and thepurpose of which is to establish contact between the two electrodes whenthe same are heated by a prolonged discharge current. Due to the highmelting point of molybdenum the risk of the sticking of the twoelectrodes because of the heat development, when the temperatureapproaches the melting point of the bimetallic components, iseliminated. The limitation imposed by bushing 7 upon movements of thebimetallic electrodes 5 and 6 prevents an increase of the air gap. Thisis important as an increase of the air gap would change the ignitionvoltage of the tube. It has been found that the contact electrodes, whenseparating after having been in contact with each other due tooverheating, tend to return into positions in which the air gap isincreased. Such return beyond the initial position is prevented by thebushing. In order to reduce the ignition delay of the tubes, theelectrodes are coated with some weakly radioactive activating means 10,ll, but this alone is not sufiieient to obtain the desired functioningof the tube. To attain the desired result, a light metal powder, forinstance aluminum powder 12 is blown into the tube before mounting theelectrodes. This powder, due to the attraction force and the eventuallyexisting electrostatic charges, is drawn to and deposited on the insidewall of the tube. Due to the tendency of the individual particles toadhere themselves to the inside wall of the tube in a dispersedrelationship and due to the automatically forming surface oxidation ofthe aluminum particles, aluminum oxide being a non-conductor, theparticles will be, generally speaking, insulated from each other againstthe electrical potentials which are normally always present betweenlines in operation. As a result, minute sparking gaps between theparticles will be formed in which discharges will appear when thevoltage difference between the electrodes is suddenly increased aboveits normal value. Due to the random distribution of the particles on theinside wall of the tube envelope, one or several of these dischargeswill always appear in a convenient location relatively to the maindischarge gap between the tube electrodes, thus effectively contributingto the ionization of said discharge gap and, consequently, to areduction of the ignition delay of the tube.

During the manufacture of an overvoltage protector or arrestor finealuminum powder is blown into the finished tube envelope which at oneend is connected to a gas pumping tube, whereby the different particlesbecause of their insignificant weight and mass are drawn to the envelopeand adhered to the inner wall thereof without the use of any specialbinding means. Thereafter the internal assembly of the tube, consistingof the ceramic bushing wherein the electrodes and the conductors aresecured, is inserted, and the soldering of the tube end is carried outin such a way that said conductors pass through the base of the tube toserve as contact tags. The tube is subsequently evacuated, whereafterthe inert gas filling is provided, whereupon the final soldering iscarried out.

The invention is not limited to the example given in the description,several modifications being possible within the scope of the invention.

I claim: 7

1. An overvoltage arrestor comprising a sealed glass tube, two lead-inwires sealed through the base of the tube, two bimetal contact stripsextending from the lead-in wires substantially in parallel relationshipinside the tube, a gaseous filling in said tube, and a light dispersionof a fine metal dust loosely adhering to the inside walls of the tube toform a multitude of small air gaps between the dust particles.

2. An overvoltage arrestor according to claim 1, wherein the bimetalcontact strips are provided with an activating, weakly radioactivecoating on areas facing each other.

3. An overvoltage arrestor according to claim 1, comprising limit meansfor limiting movement of the bimetal strips, said limit means being inthe form of a substantially cylindrical insulation bushing closelyencompassing the lower part of said strips.

4. An overvoltage arrestor according to claim 3, where in the bimetalcontact strips are provided with an activating, veakly radioactivecoating on areas facing each other.

5. An overvoltage arrestor according to claim 1, wherein said metal dustconsists of discrete particles of oxidized aluminun References tCited inthe file of this patent UNITED STATES PATENTS 2,608,545 Dobke et al July16, 1935 2,062,663 Kautter et a1 Dec. 1, 1936 2,259,111 Laidig Oct. 14,1941 2,457,487 Peacock et al. Dec. 28, 1948 2,741,680 Lemaigre-VoreauxApr. 10, 1956

1. AN OVERVOLTAGE ARRESTOR COMPRISING A SEALED GLASS TUBE, TWO LEAD-INWIRES SEALED THROUGH THE BASE OF THE TUBE, TWO BIMETAL CONTACT STRIPSEXTENDING FROM THE LEAD-IN WIRES SUBSTANTIALLY IN PARALLEL RELATIONSHIPINSIDE THE TUBE, A GASEOUS FILLING IN SAID TUBE, AND A LIGHT DISPERSIONOF A FINE METAL DUST LOOSELY ADHERING TO THE INSIDE WALLS OF THE